1. Field
The present invention is related to detection of pilot signals by a digital cellular communications mobile unit. Specifically, the present invention relates to a method and apparatus of using a matched filter in a digital cellular communications mobile unit to search for and detect pilot signals generated by cellular base stations.
2. Background
In a code division multiple access (CDMA) spread spectrum communication system, a shared frequency band is used for communication with all base stations within that system. An example of such a system is described in the TIA/EIA Standard TIA/EIA-95-B entitled “Mobile Station-Base Station Compatibility Standard for Dual Mode Wideband Spread Spectrum Cellular System,” incorporated herein by reference. The generation and receipt of CDMA signals is disclosed in U.S. Pat. No. 4,901,307 entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEMS USING SATELLITES OR TERRESTRIAL REPEATERS” and in U.S. Pat. No. 5,103,459 entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM,” both of which are assigned to the assignee of the present invention and incorporated herein by reference.
Radio Frequency (RF) signals are exchanged between a respective mobile unit and one or more base stations. Mobile units do not communicate directly with one another. Base stations communicate with a base station cellular or personal communication system controller, referred to herein as a base station controller (BSC) using various media such as ground based wires or a microwave link, for example. The BSC can route calls to a public switching telephone network (PSTN) or can route packets to a packet switched network, such as the Internet. The base station also coordinates the operation of base stations within the system during soft handoff for example.
TIA/EIA-95 is one example of a CDMA communication system. Communication from a mobile unit to one or more base stations in a TIA/EIA-95 CDMA system takes place over shared frequency channels each of which occupies approximately 1.25 MHz of radio frequency bandwidth. More specifically, communication signals occupying a given frequency band are discriminated at a receiving station through the spread spectrum CDMA waveform properties based on the use of a high rate pseudonoise (PN) code. A PN code is used to modulate signals transmitted from the base stations and mobile units. Signals from different base stations can be separately received at a given mobile unit either by the discrimination of different PN codes, and/or by the discrimination of shifted versions of the same PN code. The high rate PN spreading also allows a receiving station to receive a signal from a single transmission station where the signal has traveled over distinct propagation paths. Demodulation of multiple signals is disclosed in U.S. Pat. No. 5,490,165 entitled “DEMODULATION ELEMENT ASSIGNMENT IN A SYSTEM CAPABLE OF RECEIVING MULTIPLE SIGNALS” and in U.S. Pat. No. 5,109,390 entitled “DIVERSITY RECEIVER IN A CDMA CELLULAR TELEPHONE SYSTEM,” both of which are assigned to the assignee of the present invention and incorporated herein by reference.
The various channels within a given “forward” (base station to mobile unit) TIA/EIA-95 CDMA channel include data channels, a synchronization channel, a pilot channel, and a set of paging channels, all transmitted from the base station to mobile units. The pilot channel carries a reference signal, commonly known as the pilot signal. The pilot signal is a regularly repeated digital pattern of “chips,” wherein each chip is represented by a single binary digit. In the exemplary embodiment, the pilot signal is a pattern that is 32,768 “chips” in length, which repeats at a chip rate of 1.2288 MHz. Thus, the pattern repeats itself every 26.6 milliseconds (ms).
The pilot provides for time reference and for amplitude and phase tracking. The pilot signal allows mobile units to identify and become synchronized with the relative phase of a base station that is within range of the mobile units' communication capability. Synchronization with a base station allows the mobile unit to further refine its timing and receive data signals from the base station.
However, as the mobile unit moves, the base stations with which it is synchronized may become more distant or become blocked, and the signal from various stations may become too weak for continued reception. Further, as the mobile unit moves, a closer base station that was previously blocked may become unblocked. The more powerful signal from the closer base station may then suppress the reception of the weaker signal from the more distant, synchronized base station.
Accordingly, a mobile unit must periodically perform searches for pilot signals transmitted from other, alternative base stations in order to identify a base station with a stronger or higher power pilot signal with which to synchronize. In general, in order to facilitate these searches, the synchronized base station sends signals to the mobile unit that identify phase offsets of pilot channels for base stations neighboring the synchronized base station. Typically, to avoid pilot signal overlap, the pilot signals of neighboring base stations are phase shifted by at least 64 chips from each other. Thus, if a mobile unit is synchronized with a base station transmitting a pilot signal at a relative phase shift of 128 chips, the synchronized base station could have neighboring base stations broadcasting at relative phase shifts of 64 chips, 192 chips and, perhaps, 256 chips, for example. The mobile unit can then search for neighboring base station pilot signals around the specific phase offsets identified by the currently synchronized base station (e.g. 64, 172 and 256) on a periodic basis to determine whether to synchronize with another base station.
FIG. 1 is a block diagram of an earlier signal detection circuit or “searcher”10 that can be used in a mobile unit to check the power of pilot signals at certain given phase offsets or to search for received pilot signals over an entire sequence of phase offsets. Searcher 10 includes a despreader 12, a correlator 14, an energy storage and sorting module 16, and a processing control 18.
The base station creates a pilot signal having two components: an in-phase, or “I” component; and a quadrature, or “Q” component. Using these two components, the base station modulates or “spreads” the pilot signal. Most often, the specific protocol used in spreading a CDMA signal is referred to as Quadrature Phase Shift Keying (QPSK) spreading. QPSK spreading is discussed in detail, for example, in R. Prasad, CDMA for Wireless Personal Communications, (Artech House, 1996). After receiving a signal and passing the signal through an analog to digital converter (not shown), despreader 12 performs a mathematical algorithm on the I- and Q-components of a signal to ensure that the correct signal magnitude is detected. The mathematical algorithm used for PN despreading involves the exclusive-oring (XORing) of expected I- and Q-components with the I and Q components received. The specifics of the mathematical algorithm, as well as the specific components used in a typical despreader are well known in the art.
Correlator 14 compares an input despread signal from despreader 12 and compares it with a reference signal, commonly termed an expected signal. The expected signal can include a portion of the 32,768 chip pattern of the PN pilot signal provided to the correlator at a certain phase offset. Correlator 14 produces an energy output indicative of the level of correlation between the input despread signal and the reference signal. For example, while an exact match of all compared chips will yield a high energy output, and a match of 50% or less of all compared chips will yield a low energy output, various energy outputs between the high and low range will be yielded in accordance with a match that falls between these levels.
FIG. 2 is a schematic diagram of a greatly simplified correlator 14. A detected signal is input into a comparator 112 at input 114 and an expected signal is input into comparator 112 at input 116. For purposes of the present application, it is convenient to discuss the signal transmitted by a base station as a digital signal composed of 1's and −1's. The comparator 112 can perform, for example, a multiplying function such that if the digital components of the signals at inputs 114 and 116 match (e.g. 1,1 or −1,−1), the output 118 is 1, or high, and if the digital components of the signals at inputs 114 and 116 do not match (e.g. −1,1 or 1,−1) the output 118 of comparator 112 is −1, or low.
Output 118 is then fed into integrator 120 which sums the outputs of comparator 112 over the total period of the portion of the PN signal used as the expected signal. For example, if the expected signal is 1024 chips in length, then the integration time will be (1024)(0.81 microseconds)=829 microseconds, because a chip arrives every 0.81 microseconds in the preferred embodiment. In this way, the output 14 of the correlator will have a relatively large magnitude when the detected signal matches the expected signal.
Additionally, even when there is a match of the pilot signal with the expected signal, the magnitude of the correlator output will be larger for a stronger signal. This is because as a signal becomes weaker, that is, if it is transmitted over a relatively long distance or reflected off various objects, it degrades. This degradation results in changes of the individual chip values of the pattern of the transmitted pilot signal. Thus, for a weaker signal, even though the pilot signal is being transmitted at a phase offset that is being searched for, it is likely that fewer chip values will result in matches, and the integrator of the correlator will therefore not sum as many positive values as would be summed were the signal stronger. Thus, the output of the integrator is a relative representation of the strength or energy of a given phase pilot signal that was searched for. The larger the magnitude of the correlator output 14, the stronger or higher energy of the detected pilot signal.
The output of the correlator 14 is fed into energy storage and sorting module 16. Energy storage and sorting module 16 can perform a number of functions. It can place in memory output energies from correlator 14 that are above a certain threshold. It can also place in memory the corresponding phase offset of the pilot signal that generated the stored energies. Alternatively, it can place in predetermined memory locations corresponding to given pilot phase offsets (such as offsets corresponding to neighbors of the synchronized base stations) the energy output of the correlator 14 at that phase offset. Specific components used for, and the operation of, typical energy storage and sorting modules are well known in the art and generally include RAM for storing energy levels and phase offsets, and logic for sorting the different energy levels.
Processing control 18 provides general control for the mobile unit and makes determinations concerning with which pilot signal, and therefore which base station, the mobile unit should synchronize. It can access the energy information in energy storage and sorting module 16 for data on which to base such determinations. Processing controls such as processing control 18 typically include a microprocessor, memory, and busses, the configuration of which is well known in the art.
Because processing control 18 provides general control for the mobile unit, it may only be able to allocate a small amount of time to checking the energy storage and sorting module 16 to determine if the mobile unit should be re-synchronized with another base-station. Additionally, the sequential collection of information concerning magnitudes of pilot signals having phase offsets other than that of the currently synchronized pilot signal can be time consuming. Thus, from time to time, the mobile unit may remain synchronized with a pilot signal of a given base station even though it could synchronize with the pilot signal of another closer, or otherwise more suitable, base station instead. This can undesirably lead to the signal jamming or weak signal difficulties described above.
Additionally, due to reflections off of buildings, hills or other obstacles and/or atmospheric conditions, the neighboring base stations may not appear to be phase shifted by exactly 64 chips from the synchronized base station. And, depending on the location of the synchronized base station, there may be other base stations in the area for which phase offsets were not provided by the synchronized base station. Thus, it may be desirable to search the full sequence of 32,768 phase offsets for pilot channels, not just pilot channels that are phase shifted by predetermined multiples of 64 chips from the currently synchronized base station. The searching, sorting, storing and processor control checking associated with such full sequence searches can consume considerable processing control resources.
Methods and apparatus are also known for determining when to handoff a signal from one base station to another as a mobile unit moves. One such method using a matched filter is disclosed, for example, in U.S. Pat. No. 5,864,578, to Yuen, et al., for a “MATCHED FILTER BASED HANDOFF METHOD AND APPARATUS.” However, the matched filter disclosed by Yuen must be programmable. Such a programmable matched filter can be relatively expensive to manufacture and can require a relatively large amount of power to operate and require extra circuitry. This can be disadvantageous as mobile cellular units typically operate with a limited power supply battery and have limited space for circuitry.
Accordingly, cellular pilot searching improvements are desired. In particular, such searching should be able to be performed relatively quickly while still encompassing a broad portion of the full PN pilot sequence. A searcher should also allow for relatively high frequency checks of high priority pilot phase offsets, such as those of base stations that neighbor the synchronized base station. Additionally, such searching should require a relatively small amount of processing control time to monitor. Also, the searcher should be relatively low cost to manufacture, consume relatively low power, and be relatively compact.